This invention relates to a television video signal processing system, and more particularly to a video signal processing system suitable for a wireless pay-television system in which, to provide special programs only to subscribers who make regular payments for the service, the programs are broadcast in coded or scrambled form requiring a decoding or unscrambling device at the receiver to reproduce the programs on a common TV-set.
A so-called wireless pay-TV system, to which the present invention is applicable, has been developed to eliminate annoying commercial messages indispensable to ordinary commercial broadcasting and possible degradation of program quality due to various restrictions inherent to the commercial broadcasting. This wireless system is expected to be spread wide because expenses and time required for laying cables can be curtailed and there is no limitation in number of subscribers.
In general, in the wireless pay-TV system, a video signal and/or an aural signal is coded or scrambled so that the system may be applied only to the subscribers of the system, excluding non-subscribers.
FIG. 1 shows a television signal processed according to a conventional coding system. In this coded television signal, synchronizing signals are inverted with reference to a pedestal level, horizontal synchronizing signals are reduced, leaving one for every several lines, and amplitude modulation is effected in such a polarity that power is increased in a direction of a white level of a video component.
When non-subscribers receive the aforesaid coded signal with negative-modulation television receiver, a synchronizing separation circuit of the receiver detects a peak portion of the signal but it also detects a peak portion of a video component at the interval where horizontal signals are removed. Therefore, horizontal synchronism at the receivers becomes poor and a normal picture can not be obtained.
This conventional processing system, however, has a problem in that the amplitudes of the inverted synchronizing signals should be larger than that of the video component. This restriction is based on such an idea that a difference in amplitudes between the synchronizing signals and the video signal is utilized for separation of the synchronizing signals in the course of decoding the coded television signal into a normal television signal. In this connection, it is to be noted that a synchronizing separation circuit utilizing a differential amplitude is now widely used in television receivers and that the synchronizing signals can be separated from the coded television signal using such a synchronizing separation circuit. As a result, the coded signal can be decoded into the normal television signal. Thus, there is a serious problem of tapping by the non-subscribers.
In the conventional television signal processing system, when the picture becomes dark, i.e., when the amplitude of the video component becomes small, the differential amplitude between the synchronizing signals and the video component becomes so large that ordinary household television receivers can effect selective separation of the horizontal synchronizing signals. In such a case, horizontal synchronism is effected normally, so that the picture can be perceived legibly.
As described above, the conventional system is not sufficient and involves a problem of tapping, and therefore is not suitable for a pay-television system.